1. Field of the Invention
The present invention relates to a differential amplifier comprising an amplifying circuit for amplifying and outputting a difference voltage between a non-inverting input node and an inverting input node, and further comprises an output buffer circuit for outputting a signal based on the output of the amplifying circuit. More particularly, the present invention relates to a differential amplifier having a protection circuit that protects an output buffer circuit when its output node is short-circuited to a high potential supply node, a low potential supply node, or a low impedance portion, such as an output node or the like, of some other circuit.
2. Background Art
In any known differential amplifier having an amplifying circuit and an output buffer circuit, when an output node of a load-connected output buffer circuit is short-circuited to a high potential supply node, a low potential supply node, or a low impedance portion, such as an output node or the like, of some other circuit, there exists a danger that an excess current will flow in the output stage of the output buffer circuit.
In order to eliminate such a danger, an improved differential amplifier may be contrived, as shown in FIG. 5, for example, incorporating a protection circuit. In FIG. 5, there are included an amplifying circuit 21 driven by a supply voltage and amplifying a difference voltage between a non-inverting input node and an inverting input node; an output buffer circuit 22 driven by the same supply voltage as applied to the amplifying circuit 21; a sense resistor 23 for converting into a voltage the current flowing in the output stage of the buffer circuit 22; and a comparator 24 for comparing the voltage generated across the sense resistor 23 with a reference voltage and, in response to an increase in the voltage across the sense resistor 23 beyond the reference voltage, interrupting or limiting a base current supplied to a transistor or transistors constituting the output stage of the buffer circuit 22.
In such a protection circuit, the current flowing in the output-stage transistor of the output buffer circuit 22 is converted by the sense resistor 23 into a corresponding voltage and detected to determine whether the voltage is higher than a threshold voltage. If the voltage is higher than the threshold voltage, the current flowing in the output-stage transistor of the output buffer circuit 22 is interrupted or limited.
More specifically, when a short-circuit occurs between the output node of the output buffer circuit 22 and the high potential supply node, then a current flows from the high potential supply node, via the output node of the output buffer circuit 22, the output-stage transistor thereof, and the sense resistor 23, to the low potential supply node (GND). At this time, the voltage generated across the sense resistor 23 is determined by the current flowing in the sense resistor 23 and the sense resistance (=sense voltage).
If the sense voltage is higher than a threshold voltage (reference voltage), the comparator 24 interrupts or limits the base current supply (output pull signal generating circuit) to the output-stage transistor of the output buffer circuit 22, thereby preventing any excess current from flowing into the output of the buffer circuit 22.
However, when the current flowing into the output-stage transistor of the output buffer circuit 22 is reduced, the current flowing in the sense resistor 23 is also reduced and lowers the voltage generated across the sense resistor 23. Then, the comparator 24 halts interruption or limiting of the base current supply (i.e., an output pull signal generating circuit) to the output-stage transistor of the output buffer circuit 22, thereby allowing the current to flow again into the output-stage transistor of the output buffer circuit 22. This action is repeated during the short-circuited state.
Since the sense resistor 23 is connected in series to the output-stage transistor of the output buffer circuit 22, the dynamic range at the output node of the output buffer circuit 22 is narrowed by a value corresponding to the sense voltage generated across the sense resistor 23.